Mobile telephone systems have become very popular in recent years to the point where they are preferred over standard landline telephone systems by individuals and enterprises alike. One problem with mobile telephone systems is that they rely on cells extending over well-defined geographic areas traditionally served by a main base station. However, service for devices operating at the fringe of a cell is frequently intermittent or has a low quality. Moreover, as more and more devices are active within a cell, the cell's capacity to service the devices is strained and communication with the devices becomes poor. A further disadvantage is that because of local conditions within a cell area (for example, inside a building, or in an underground location), communications with certain devices may be severely attenuated and it is difficult if not impossible to service such devices from the main base station.
Moreover, some entities, including enterprises, may find it desirable to control communications with certain devices within a limited section of the cell, for example, inside a factory, or other facility. The standard protocols used for communications within a traditional cell area do not permit such functionality.
One solution to reduce these problems involves providing one or more small cells that are disposed physically either within a main cell or at its fringe. Each small cell is provided with a low power local base station communicating with the main base station via DSL, cable or broadband or other conventional communication channels. Devices within the small cell normally communicate with the local base station and not the main base station. Importantly, all communications between the devices and both the main and the local base stations are performed within the same frequency channels using a conventional protocol, such as WDCMA, GSM, CDMA2000, TD-SCDMA, WiMax, LTE, etc. Small cells can be categorized based on their operating range as femtocells (with a range of about 10 m), picocells (with a range of about 10-200 m) or microcells (with a range of 200-2000 m). In this environment, a main cell is termed a macrocell. The type of small cell depends obviously on the size of the respective area that must be serviced by it. For example, picocells are suitable for residences or a floor in a multi-storied building.
Small cell service is available from many mobile operators, such as Vodaphone, AT&T, Sprint, Verizon, etc. A small cell service is advantageous to a mobile operator because it improves the coverage and increases the capacity of the respective macrocell. Consumers additionally benefit from a small cell because they get better voice quality; an increase in the battery life in their devices because of lower power requirements, and can use them to define a closed network.
Small cells can be operated in an open mode in which the small cell is available to any device within its coverage area, or in a closed mode in which devices must be registered or subscribed with the local base station before they can operate with the small cell. In other words, in an open small cell, a device within the small cell coverage area is serviced by the local small cell base station. As the device moves out of the small cell into the macrocell, it is seamlessly handed off to the main base station. An unsubscribed device in a closed small cell is ignored by the local base station and must be serviced by the main base station, if possible.
A problem with a mobile telephone system with a macrocell and one or more small cells is that the imbalance between the EIRP (Effective Isotropically Radiated Power) of devices served by the different base stations can cause serious interference between the two devices. Until now, this interference could be reduced only by reducing the range of the small cell so that both devices are served by the main base station. Obviously this is unsatisfactory because it defeats the purpose of defining a small cell.
In a picocell, this interference may be very pronounced, appearing as a mutual interference resulting in a poor signal-to-noise ratio during downloads to a device as well as uploads to the main base station.
Similarly, an unsubscribed device within a femtocell must be serviced by the main base station. This causes interference both for downloads by other subscribed devices and uploads to the local base station.
The present disclosure solves these problems, so that the benefits of small cells are not lost.